1. Field of the Invention
Embodiments of the present invention generally relate to downhole sensing and, more particularly, to optical couplers used in a downhole splitter assembly.
2. Description of the Related Art
The world's reservoirs are aging. This translates to increased water production and gas coning, increased lifting costs, expensive treatment of produced water, and high cost of deferred or lost hydrocarbon production. Hence, it is becoming increasingly important to accurately measure and understand conditions inside a well, reservoir, or field. Downhole sensing offers measurement near the areas of interest—e.g., near the wellbore or reservoir—and thus offers potential for higher quality data, more insight across a sandface, and measurement of parameters that are not available on the surface. This information can be used to optimize production, locate water or gas coning, manage fractures or fluid movement in the reservoir from seismic disturbances, etc.
In the hydrocarbon industry, there is considerable value associated with the ability to monitor the flow of hydrocarbon products in the production pipe of a well in real time. For example, formation properties that may be important in producing from, injecting into, or storing fluids in, downhole subsurface reservoirs comprise pressure, temperature, porosity, permeability, density, mineral content, electrical conductivity, and bed thickness. Further, fluid properties, such as viscosity, chemical elements, and the content of oil, water, and/or gas, may also be important measurements. Downhole properties may be measured by a variety of sensing systems including acoustic, electrical, magnetic, electro-magnetic, strain, nuclear, and optical based devices.
Many optical components have a characteristic wavelength that may be found by interrogating the optical component with an optical source capable of producing light at various wavelengths over a fixed range or bandwidth. For example, fiber Bragg gratings (FBGs) (typically formed by photo-induced periodic modulation of the refractive index of an optical waveguide core) are highly reflective to light having wavelengths within a narrow bandwidth centered at a wavelength generally referred to as the Bragg wavelength. Because light having wavelengths outside this narrow bandwidth is passed without reflection, Bragg wavelengths can be determined by interrogating a Bragg grating with a light source swept across a bandwidth that includes the Bragg wavelength and monitoring the reflected optical power spectrum at a receiver unit. Because Bragg wavelengths are dependent on physical parameters, such as temperature and strain, Bragg gratings can be utilized in optical sensor systems to measure such parameters.
In these and a wide range of other types of optical systems, the measurement of a characteristic wavelength of an optical component to great accuracy (and/or with great repeatability) is important to system performance. Two significant parameters determining the error of any such measurement are the signal-to-noise ratio (SNR) and effective integration time of the measuring system. SNR is dependent of many factors including received optical power, optical-source noise, and receiver noise. The effective integration time is dependent on overall averaging time and the proportion of that time which is producing useful signals at the receiver unit. Improving these two parameters can improve characteristic wavelength measurement repeatability and accuracy.